Lua_api.txt: Add chat command params info

doc/lua_api.txt

@@ -5156,6 +5156,17 @@ Definition tables
                                       -- Returns boolean success and text output.
     }
 
+Note that in params, use of symbols is as follows:
+
+* `<>` signifies a placeholder to be replaced when the command is used. For example,
+  when a player name is needed: `<name>`
+* `[]` signifies param is optional and not required when the command is used. For
+  example, if you require param1 but param2 is optional: `<param1> [<param2>]`
+* `|` signifies exclusive or. The command requires one param from the options
+  provided. For example: `<param1> | <param2>`
+* `()` signifies grouping. For example, when param1 and param2 are both required,
+  or only param3 is required: `(<param1> <param2>) | <param3>`
+
 ### Detached inventory callbacks
 
     {